1. Too many people in a harem...I am fine with a love triangle, but when you have more than two competing over the same person, it gets a little irritating.

2. I'm not a prude by any means, and don't mind smut or even hentai if the storyline with it is good. Most the time this isn't the case - sometimes I keep reading to see if the story is going anywhere...but usually there is no specific plot and the manga plods along to an uneventful or even weird ending.

3. There was a major battle and everything finally came to a head and the main characters win, and then "oh crap" something else happens immediately and we are off on a completely new storyline...no downshift what-so-ever (I have put The World God Only Knows on the back burner for now).

4. You were waiting the whole entire series for the main characters to get together, but it keeps dragging on and on for chapter after chapter...I keep plodding along with it anyways usually, but then the ending just is a bid let down (talking about you Faster Than a Kiss).

5. The main character is a baby or the manga is centered around children age 5 or less...just can't find enjoyment in that. Maybe if I had kids of my own, I would see the cuteness, but then again if I had kids, I probably wouldn't have much time to read manga in the first place...

5. Constantly uses fanservice as a mean to get fans to like this new female character. I don't mind it as long as it doesn't effect the pacing. (For example, Fairy Tail)

4. The bad guy who tormented the main character since they were young and also kills off one of their childhood friends, turns good and is liked by said tormented character. (Again Fairy Tail, I'm sorry but that was just stupid.) Not to mention Erza being my favorite character and Jellal being my most hated character in anime/manga history.

2. No movement in plot/boring plot, and it's not an episodic series like Gintama.

1. A good storyline turns incredibly cliche/too mainstream.
For me, Psyren had a great start, but then the "main bad guys" showed up and it became just another battle manga. Same happened to Medaka Box too. And most shojo/harem manga. D: All it does is repeat what every other series does. OTL I ended up dropping most of the top shonen manga out there too because it started getting repetitive - Bleach, Reborn, Fairy Tail, Beelzebub, Blue Exorcist. Shojo romance is even worse. Most of the time I won't even remember shojo manga after I've read them. Only the ones with really distinctive plots or interesting side features stay in my memory - Oresama Teacher, Arisa, Skip Beat and Seiyuu Ka.

Nothing has ever stopped me from reading a manga, with the exception of me getting sidetracked with another manga, anime or game.

I'm sad when a main or side character dies, but you have to pull through it. Don't care about there being too much ecchi, I mean it's usually their as comic relief rather than something to move the plot on.

Anyway, I never go off an anime for stupid reasons like a character dying, too much ecchi (is that possible) or even clichéd story. Only boredom is the main determaining factor in whether it keeps my attention.

1.If the heroine just KEEPS HITTING the main character for everything he does or if she just hits the main character although he said that it is a misunderstanding..I would just call the police, cause she just can't understand until shit hits the van, than she always goes ''sorry, I'm soooo sorry''..Like dude you kept hitting me half to death and now you just say ''sorry'' ?A sorry isn't going to cut it.
2.If there is too much Ecchi that disturbs my reading..
3.If the Main character is a pussy who can't stand for himself, without getting stronger or when he is strong but keeps being a pussy..
4. If the main character is a girl (They tend to have personalities i just can't like.)..
5.If the story is boring..

I try my hardest to make it at least through the first volume of a book but, its not always easy.

1.) I absolutely HATE it when the main character or one of the important characters is the most wishy washy, useless female to ever be created. And its not just Inoue from bleach, she can be kind of a slight improvement from the crap I've blocked out of my mind. Whenever the story has a hint of "i know I'm weak and useless but I can't leave you alone so I, without any combat training or specialized knowledge in what ever needs to be done will follow you (most likely Blindly) into a very dangerous situation. Which I know you can handle alone, but I want to be near you, so you will get unnecessary injuries trying to protect little old me" B!t@H!!!! Stat Back! Grrr!!! To be clear, This scenario is acceptable " I have no combat training, but i have specialized knowledge that is needed, I am a wimp but even wimpy me can't let him go into an unknown situation. I need to woman up and help, but scared. Will go anyway" This last scenario can lead to more character development because it effects all character. But The first example GRRR!!!!!!!!!!!!!!!!!. I'm done.
2.) Wolf Guy... Need I say more? I'm so hurt. I want the time I spent reading to the end back. I want it back so bad. I never been deceived so hard before. For those who don't know, this is an example of a plot going from "This is soo totally freaking AWESOME!!!" to " WTF!! Are you serious, wait...WHAT!! Da Hell!!
3.) Very predictable story with nothing new. A lot of stories are predictable, but they can bring their own flare. But just as many refuse to be innovative. I'm looking at you shoujo genre. Like seriously. Yaoi and Shonen are leaps and bounds ahead of shoujo in variety when it comes to plot, character development, everything IMO.
4.) Too Much fan service. I'm between a rock and a hard place. Since I tend to read more shonen, than seinen or any other genre, I'm constantly running into panty shots, suggestive poses, and suggested incest usually initiated by the sister.(sigh) This is usually where I again curse shoujo for being so mediocre and curse myself for not knowing enough seinen titles to run to. Then I curse the author for having a good plot that keeps me reading until the fan service wins and i drop the manga all together.

Ok, I'm done ranting
All these can be applied to how I watch anime as well.

I haven't read a lot of manga, but it seems pretty common based on what I have read for stories to move really slowly, and not in any deep sort of way. When that happens, I get bored, and drop it. Especially if I know there are lots and lots more volumes (if I know it will end soon, I'll stick with it to the end.)

1. No character development (or if it has, it's at a snail's pace)
2, Bad art (can be so distracting for me, since I'm looking at it 90% of the time)
3. Repetitive story lines. Same issue will pop up X number of chapters later.
4. Plot going in circles to milk out its popularity that the story is just a mere shadow of what it was in the first 5 volumes
5. "Rape and violence is love" themes.

The same reasons I would stop reading a book. But a few are, for the most part, unique to manga.

1) Slow progress is a major reason for me to give up. Weekly releases are wonderful and monthly releases are fine too, but when half a year passes without a story update I start to put it behind me.

2) Obnoxious use of timeskips.

3) Intentionally frustrating romances with no planned endgame. Unresolved sexual tension can keep me interested for a while, but emotionally blueballing the readership is a turnoff.
This one goes out to you, Kubo. Just hook Ichigo up with Rukia already. It's what everyone wants.

4) Shitty translations. Empty bubbles and choppy speech take away from the story.

5) Spending 20 chapters beating the same joke to death, then violating it's corpse for the next 10. Most of you probably know what I mean.